1. Field of the Invention
The present invention relates to telephone or data communication systems for connecting a plurality of remote user stations to a computer, and particularly to such systems including facilities for locating and isolating malfunctions which may occur in the systems.
2. Description of the Prior Art
Computers are often connected through telephone or data communication systems to a plurality of remote user stations by use of data bridging circuits. One such prior art system or network employs a four-wire trunk network including parallel transmitting and receiving lines to which the computer is connected at one end, and at the remote end, a data bridging circuit is utilized to join the four-wire trunk from the computer to a plurality of four-wire branch or drop channels to user stations and/or to other bridges which may in turn branch to additional user stations and/or additional bridges. The process of connecting a plurality of bridges in series is referred to as "cascading." The bridges so employed incorporate adjustable amplifiers, resistance pads, and isolation transformers to join the primary collection and distribution ports to the pluralities of secondary collection and distribution ports serving the user stations. These incorporated features are necessary to provide appropriate signal levels and signal conditioning on the primary and secondary four-wire service lines. The bridges may be located in widely separated telephone substations, switching offices, or user premises. User terminals will seize the communication system in order to communicate with the computer. The system is automatically released from seizure when individual communications are finished.
Malfunctions can occur in any of the lines. These can include malfunctions such as short circuits, open circuits, excessive noise generation; or user station equipment, such as terminals, modems, or other data transmission devices may malfunction and continously seize the network lines. This type of seizure is referred to as data streaming. When this occurs, coherent data cannot be transmitted. Further, such seizure cannot recognize an unseized condition. Such data streaming can be said to take down the entire network under those conditions. Complex efforts are required in order to correct data streaming conditions. Those efforts may extend for considerable distances through various telephone facilities. They are time-consuming efforts and may involve considerable manpower and office coordination in order to permit isolation of all user station facilities. Eventually manual testing of the individual lines is necessary to determine the source of malfunction and to take corrective action in isolating the malfunction in order to permit the restoration of network services.
The prior art utilizes remote switching and testing units which are installed at various points in the system, principally the end-user locations. These units can be signaled by an operator from respective bridge locations in order to provide isolation of the user station facilities. More sophisticated trunking capability from time to time will permit automated isolation of trunk locations but does not normally provide the ability to isolate user station facilities. One prior art user station testing capability generally consists of a loopback test circuit which is connected and disconnected from the network by a specific frequency of 2713 Hertz. This signal must be individually transmitted to each location. Since all user station terminating units respond to 2713 Hertz, it is obvious that a 2713 Hertz signal which would appear at a four-wire network would also appear at all user stations by virtue of passing through the bridge to those stations. This condition would create widespread unwanted loopback and, in fact, deter any testing capability.
Prior art examples of channel terminating units which address the difficulty of unwanted loopback are described in U.S. Pat. Nos. 3,814,840, 4,258,236, and 4,403,119. These remote testing units may each include facilities which interpret a pulse code, a single frequency code or a dual tone multi-frequency (DTMF) address code, so that an operator can establish a discrete address code for each remote unit and by applying conventional pulse, single frequency or DTMF signaling capability, address the discrete unit through a plurality of bridging circuits and network circuitry common to all units. The prior art terminating units also may include the ability to remotely control the command ability of various testing functions, such as conventional four-wire loopback, transmission of a milliwatt signal, and the creation of a quiet termination mode in the transmit path by opening the transmit line through a command switching address function.